High seminal quality is important to achieve acceptable pregnancy rate following artificial insemination with cryopreserved semen in the equine industry. Docosahexaenoic acid (DHA) is an omega-3-polyunsaturated acid, which improves the integrity of the spermatozoa membrane during temperature changes. Insulin-like growth factor-I (IGF-I) is a protein hormone that helps mainly glucose to enter spermatozoa and it is an antioxidant. The aim of this study was to assess the effect of the in vitro addition of DHA in combination with IGF-I to frozen-thawed stallion semen. Three ejaculates from each of three Irish Sport Horse stallions were collected, the gel fraction was removed and semen was diluted in a 1:1 ratio using extender, centrifuged (1.000 g) for 10 minutes at 32 degrees C and ressuspended to 100 x 10(6) spermatozoa/mL in freezing extender. Semen was cooled to 4 degrees C, packed into 0.5 mL straws, frozen and stored under liquid nitrogen at -196 degrees C. Straws were thawed at 37 degrees C for 30 seconds, semen was diluted to 25 x 10(6) spermatozoa/mL and split in four treatments adding 0 or 1 ng of DHA /mL and 0 or 100 ng of IGF-I /mL:DHA0, DHA0 + IGF-I, DHA1 and DHA1 + IGF-I. Semen was incubated at 32 degrees C and after 30 minutes, total motility (TM), rapid progressive motility (PM), viability and acrosome integrity were assessed. After 60 and 120 minutes, TM and PM were assessed again. Post-thawed PM was higher (P < 0.05) when DHA1 + IGF-I was added, but there was no effect of the addition of DHA and IGF-I to TM, viability or acrosome integrity (P > 0.05).